Holographic MEMS operated optical projectors

ABSTRACT

A method forms an image with a reconfigurable array of mirrors. The method includes configuring the array by translating some of the mirrors such that distances of the mirrors of the array from a reference plane have a non-uniform spatial distribution. The method includes illuminating the configured array with a coherent light beam such that part of the light beam is reflected off the array and is projected on a planar viewing screen.

BACKGROUND

1. Technical Field

The invention relates to apparatus and methods for projecting lightimages on reflective viewing screens.

2. Discussion of the Related Art

This section introduces aspects that may be helpful to facilitating abetter understanding of the inventions. Accordingly, the statements ofthis section are to be read in this light. The statements of thissection are not to be understood as admissions about what is in theprior art or what is not in the prior art.

One type of image projector is based on a two-dimensional (2D) array oftiltable mirrors. The 2D array is illuminated by an incoherent lightsource. In the array, each mirror reflects part of the illuminationlight in a manner that depends on the mirror's orientation. Theorientation of each mirror is controlled by a correspondingmicro-electrical mechanical system (MEMS) actuator. The MEMS actuatortilts the corresponding mirror to reflect illumination light eithertowards a viewing screen or away from the viewing screen. That is, eachMEMS actuator operates the corresponding mirror in an ON/OFF manner. Themirror is ON when it is tilted to specularly reflect a light spot to theviewing screen and is OFF when it is tilted to specularly reflect alight spot away from the viewing screen. Such tilting-mirror type imageprojectors can produce a variety of specularly reflected spot images.

BRIEF SUMMARY

Various embodiments provide image projectors that are based on coherentlight sources. Rather than an image of mutually incoherent light spots,the new image projectors can form an image by substantially constructinga coherent light wavefront.

In one embodiment, an apparatus includes a light source and areconfigurable array of mirrors and MEMS actuators. The light sourceincludes a plurality of coherent sources. Each coherent source isconfigured to emit light of a different color than the remainder of thecoherent sources. Each mirror is controlled by a corresponding one ofthe MEMS actuators that is able to translate the controlled mirror. Thelight source is configured to illuminate the mirrors with a timedivision color-multiplexed light beam.

In some specific embodiments, the apparatus is such that the lightsource is configured to vary the light beam between having a first colorand having a second color at a frequency of greater than 20 Hertz. Theapparatus may include a processor configured to operate the MEMSactuators such that the mirrors of the array have a first spatialdistribution of translations when the light beam has the first color andhave a different second spatial distribution of translations when thelight beam has the second color.

In some specific embodiments, the apparatus includes a planar viewingscreen and is configured to form an image on a viewing screen that ahuman would observe to be temporally constant and to have asuperposition of the colors of the coherent sources. The viewing screenmay non-specularly reflect light incident thereon.

In some specific embodiments, the apparatus includes a beam splitterlocated to redirect a light beam from the light source towards thereconfigurable array and a wave plate located between the beam splitterand the reconfigurable array.

In some specific embodiments of the apparatus, the diameters of themirrors are less than 20 times a wavelength of light produced by thelight source, and the apparatus includes a processor configured tooperate the MEMS actuators such that the array forms a first image of afirst color on a viewing screen and forms a second image of a secondcolor on the viewing screen such that the first and second images havethe same size.

In another embodiment, a method forms an image with a reconfigurablearray of mirrors. The method includes configuring the array bytranslating some of the mirrors such that distances of the mirrors ofthe array from a reference plane have a non-uniform spatialdistribution. The method includes illuminating the configured array witha coherent light beam such that part of the light beam is reflected offthe array and is projected on a planar viewing screen. The illuminatingmay cause the viewing screen to non-specularly reflect part of the lightprojected thereon.

In some specific embodiments, the method includes then, reconfiguringthe array by translating some of the mirrors such that distances of themirrors of the array from the reference plane have a new non-uniformspatial distribution and re-illuminating the reconfigured array with acoherent light beam of a different color such part of the light beam ofthe different color is projected on the planar viewing screen. Themethod may further include during each of series consecutive of timeperiods, repeating the acts of configuring, illuminating, reconfiguring,and re-illuminating. Each of the periods may be less than 1/20 secondslong.

In some embodiments of the method, the mirrors have largest diametersthat are smaller than 20 times a wavelength of light of the light beam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a ways of reconstructingtwo-dimensional (2D) image on a viewing screen;

FIG. 2 is a block diagram of an apparatus that reflects a coherent lightbeam to project an image on a viewing screen, e.g., according to themethod of FIG. 1;

FIG. 3 is a block diagram of an apparatus that reflects a coherent lightbeam to produce a projected image, e.g., according to the method of FIG.1;

FIG. 4A is an oblique view of an exemplary micro-electrical mechanicalsystem (MEMS)-actuated micro-mirror that can be used, e.g., in theapparatus of FIGS. 2 and 3;

FIG. 4B is an oblique view of an alternate MEMS-actuated micro-mirrorthat can be used, e.g., in the apparatus of FIGS. 2 and 3;

FIG. 5 is a flow chart illustrating a method of forming projected imagesof coherent light on a viewing screen, e.g., with the apparatus of FIG.2 or 3;

FIG. 6 is a block diagram illustrating a multi-color light source thatmay be used in some embodiments of the apparatus of FIGS. 2 and 3;

FIG. 7 illustrates a problem with using a single configuration of amirror array to form multi-color projected images;

FIG. 8 is a flow chart illustrating a method of forming multi-colorprojected images of coherent light, e.g., using embodiments of theapparatus of FIG. 2 or 3;

FIG. 9 is a flow chart illustrating a first iterative method of findinga configuration for a reconfigurable mirror array that would be suitableto phase-modulate an incident light beam so that the beam projects adesired image on a viewing screen, e.g., with the apparatus of FIG. 2 or3;

FIG. 10 is a flow chart illustrating a second iterative method offinding a configuration for a reconfigurable mirror array that would besuitable to phase-modulate an incident light beam so that the beamprojects a desired image on a viewing screen, e.g., with the apparatusof FIG. 2 or 3;

FIG. 11A-11E are block diagrams of various alternate apparatus thatmodulate a coherent light beam to project an image on a viewing screen;

FIG. 12A-12B illustrate exemplary types of spatial filtering that can beperformed in the Fourier plane of a first lens system in the imageprojection apparatus shown in FIGS. 11A-11E;

FIG. 13 is a flow chart for a method of image projection based on aone-to-one correspondence between pixels of a reconfigurable spatiallight modulator array and pixels of an image-to-be-projected, e.g., asin apparatus of FIGS. 11A-11E; and

FIGS. 14A and 14B illustrate alternate ways of operating areconfigurable spatial light modulator in the image projection method ofFIG. 13.

In the Figures and text, like reference numerals indicate elements withsimilar or the same functions and/or structures.

In the Figures, the relative dimensions of some features may beexaggerated to more clearly illustrate one or more structures orelements in the Figures.

In the various embodiments, the illustrated lens and lens systems maybe, e.g., achromatic doublets.

Herein, various embodiments are described more fully by the Figures andthe Detailed Description of Illustrative Embodiments. Nevertheless, theinventions may be embodied in various forms and are not limited to theembodiments described in the Figures and Detailed Description ofIllustrative Embodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

While an image projector based on an array of tilting mirrors canproduce a large variety of light-spot images, such an image projectormay not be efficient with illumination light. In particular, part of theillumination light is typically reflected by the mirrors away from theviewing screen and thus, is lost light. Rather than reflecting lightaway, some embodiments of image projectors described herein redistributelight on the viewing screen.

U.S. patent application Ser. No. ______ entitled “SPECKLE REDUCTION INLASER-PROJECTOR IMAGES”, by Vladimir A. Aksyuk, Randy C. Giles, Omar D.Lopez, and Roland Ryf (Docket No.: Aksyuk 46-80-11-13); U.S. patentapplication Ser. No. ______ entitled “DIRECT OPTICAL IMAGE PROJECTORS”by Randy C. Giles, Omar D. Lopez, and Roland Ryf (Docket No.: Giles81-13-15); and U.S. patent application Ser. No. ______, entitled “COLORMIXING LIGHT SOURCE AND COLOR CONTROL DATA SYSTEM” by Gang Chen, RonenRapaport, and Michael Schabel (Docket No. Chen 7-7-6) are filed on thesame date as the present patent application and are incorporated hereinby reference in their entirety.

A) Reconstructive Formation of 2D Images

FIG. 1 schematically illustrates a method for forming a two-dimensional(2D) coherent light image of a desired scene.

To form a real 2D image of the scene, coherent light source may be usedas illumination light so that the scene scatters or transmits part ofthe illumination light thereby producing an outgoing light beam. Part ofthe outgoing light beam passes through a selected optical aperture andthen, projects a 2D image on a planar viewing screen. Over the selectedoptical aperture, the outgoing coherent light beam forms a pattern ofrelative phases and amplitudes, which determines the 2D image that itwill project onto the planar viewing screen.

Over the selected optical aperture, a map of the relative intensity andphase of the electric or magnetic field of the outgoing coherent lightbeam may be made in a pixel-by-pixel manner. The availability of such apixel-by-pixel map over the selected optical aperture provides a basisfor reconstructing the projected 2D image.

In particular, to reconstruct the 2D image, a light source canilluminate a reconfigurable array of micro-mirrors with coherent light.The micro-mirrors of the reconfigurable array are positioned to reflectthe illumination light in a manner that produces an outgoing coherentlight beam with approximately the same pixel-by-pixel map over the sameselected optical aperture. That is, the reconfigurable array isconfigured to produce an outgoing coherent light beam with approximatelythe same pixel-by-pixel map of both relative phases and amplitudes overthe selected optical aperture, e.g., a cross-section of the outgoingoptical beam. If the reconstructed light beam has the samepixel-by-pixel map of the selected optical aperture as a coherent lightbeam actually scattered and/or transmitted by the desired scene, then,the reconstructed light beam will project the same image on the planarviewing screen.

Herein, a reconfigurable mirror array substantially only adjusts thephase of an incident light beam over its surface. Nevertheless, theproduced spatial distribution of phases can to a good approximation fixthe spatial distribution of both phase and amplitude over a crosssection of the outgoing light beam, which is close to the reconfigurablemirror array and far from individual micro-mirrors therein. Thus, thereconfigurable mirror array is able to a good approximation toreconstruct an image as described above over such a cross section, i.e.,the selected optical aperture. The pixel-by-pixel map may haveindividual micro-mirrors correspond to pixels or may have local disjointgroups of the micro-mirrors correspond to pixels. In the later case, thepositions of the micro-mirrors of a group define the average relativeamplitude and phase of the light on the corresponding pixel of theselected optical aperture.

B) Holographic Apparatus for Image Projection

FIGS. 2 and 3 illustrate exemplary apparatus 10, 10′ for projectingimages on a planar viewing screen 12 according to the method illustratedby FIG. 1. The apparatus 10 includes a coherent light source 14, areconfigurable mirror array 16, a digital data processor 18, and adigital data storage device 20.

The planar viewing screen 12 may be, e.g., a substantially flat surfacethat non-specularly reflects or scatters back incident light, e.g., aconventional projection screen or a white wall. Due to the viewingscreen's ability to non-specularly reflect or back scatter incidentlight, viewers, V, are able to see images projected on viewing screen 12over a wide range of viewing directions.

The coherent light source 14 includes, e.g., a light source 22 and beamexpansion optics 24. The light source 22 includes one or moreconventional visible light semiconductor lasers. The light source 22produces coherent light beam 28. The coherent light beam 28 may be,e.g., linearly polarized. The beam expansion optics 24 produces a widelaterally coherent light beam 26, e.g., a collimated light beam, fromthe light beam 28 output by the light source 22. Examples of beamexpansion optics 24 include refractive lenses, refractive lens systems,and non-planar reflective optical systems.

Part or the entire coherent light beam 26 is directed to illuminate thewhole front reflective surface of the reconfigurable mirror array 16 inboth apparatus 10, 10′. In the apparatus 10, the beam expansion optics24 directs the wide light beam 26 directly towards the reconfigurablemirror array 16, i.e., at an oblique angle. In the apparatus 10′, thebeam expansion optics 24 directs the coherent light beam 26 to anoptical beam redirector 30, which in turn redirects the coherent lightbeam 26 to be substantially normally incident on the reconfigurablemirror array 16. Optical beam redirector 30 may be, e.g., a polarizationbeam splitter that is oriented with respect to the polarization of theoptical beam 26 so as to redirect light most or all light thereintowards the reflective surface of the reconfigurable mirror array 16. Insuch embodiments, the apparatus 10′ may also include a quarter waveplate 32 as shown in FIG. 3. The wave plate 32 is a birefringent platewhose produces a retardation of about ¼ of a wavelength between the twoorthogonal linear polarization components of the optical beamtransmitted normally there through. The wave plate 32 is located betweensuch a polarization beam splitter 30 and the reconfigurable mirror array16. The wave plate 32 has a thickness suitable to cause light that isreflected by the reconfigurable mirror array 16 to arrive at thepolarization beam splitter 30 with an approximately appropriate linearpolarization for transmission there through, i.e., instead beingreflected back along the incident coherent light beam 26.

The reconfigurable mirror array 16 includes a regular or irregular 2Darray of MEMS-actuated micro-mirrors 34, i.e., the micro-mirrors 34 havea substantially uniform spatially distributed along one surface of thereconfigurable mirror array 16. The micro-mirrors 34 may besubstantially identical and may have a circular, rectangular,triangular, or square shape or may have a non-symmetric shape. Eachmicro-mirror 34 is physically connected to and controlled by acorresponding MEMS actuator, i.e., so that the reconfigurable mirrorarray 16 includes a 2D spatial array of micro-mirrors 34 and acorresponding 2D spatial array of MEMS actuators. The MEMS actuators arefabricated on a planar substrate 35, i.e., a silicon substrate. The MEMSactuators provide for the ability to independently control themicro-mirrors 34 such that each micro-mirror 34 can be translated normalto a reference plane 36, e.g., normal to a surface of the planarsubstrate 35 or normal to an average reflective surface of thereconfigurable mirror array 16.

FIG. 4A illustrates one exemplary embodiment 34A for the MEMS-actuatedmicro-mirrors 34 of FIGS. 2-3. The micro-mirror 34A includes a controlcapacitor (CC), a restoring spring (RS), and a top planar mirror surface(MS) that forms the reflective surface, e.g., a silicon or metallicplanar surface. The control capacitor CC includes a moveable conductingplate (MCP) and a fixed conducting plate (FCP), which is directly fixedto planar substrate (PS), e.g., the planar substrate 35 of FIGS. 2 and3. The restoring spring RS rigidly fixes to the moveable conductingplate, MCP, via a post, P, and provides a translational restoring forcethat returns the mirror surface MS to its initial position when thecontrol capacitor CC is discharged. The moveable conducting plate MCPtranslates the mirror surface MS in a piston like motion that may be,e.g., substantially perpendicular to the surface of the planar substratePS so that the orientation of the mirror surface MS does not changeduring such motion. The piston-like translation movements of the mirrorsurface MS and the final rest positions thereof are controlled byvoltages applied across the conducting plates MCP, FCP of the controlcapacitor CC.

FIG. 4B illustrates another embodiment 34B for the MEMS-actuatedmicro-mirrors 34 of FIGS. 2-3. The micro-mirror 34B includes a controlcapacitor (CC′), a restoring spring (RS), and a top planar mirrorsurface (MS), e.g., a silicon or metallic planar surface. The controlcapacitor CC′ includes a moveable conducting plate, MCP′, and a fixedconducting plate, FCP′, i.e., fixed to planar substrate, PS, e.g.,planar substrate 35 of FIGS. 2 and 3. The restoring spring RS rigidlyfixes to the flat structure for the mirror surface MS via a post, P, andfixes to the underlying moveable conducting plate, MCP′ via another postP. Thus, the restoring spring RS is located outside of the controlcapacitor CC′ so that its presence and its position do not substantiallyeffect the electro-static forces between the plates MCP′ and FCP′. Itmay be desirable to have each restoring spring, RS, of the micro-mirrorslocated outside of the paired plates MCP′ and FCP′ to avoid interferencewith the electrical control of the control capacitors CC′. The restoringspring RS provides a translational restoring force that returns themirror surface MS to its initial position when the control capacitor CC′is discharged. The moveable conducting plate MCP′ translates the mirrorsurface MS in a piston motion substantially perpendicular to the surfaceof the planar substrate PS so that the orientation of the mirror surfaceMS does not change during such motion. The piston-like translationmovements of the mirror surface MS and the final rest positions thereofare controlled by voltages applied across the conducting plates MCP′,FCP′ of the control capacitor CC′.

Other exemplary MEMS-actuated micro-mirrors and/or 2D arrays thereof maybe described, e.g., in one or more of U.S. patent application Ser. No.11/009,447 filed Dec. 10, 2004 by Vladimir A. Aksyuk et al, U.S. patentapplication Ser. No. 10/813,951 filed Mar. 31, 2004 by Vladimir A.Aksyuk et al, and U.S. patent application Ser. No. 11/140,313 filed May27, 2005 by Vladimir A. Aksyuk et al. These U.S. patent applications areincorporated herein by reference in their entirety. The reconfigurablemirror array 16 of FIGS. 2 and 3 may include micro-mirrors and arraysthereof, which are fabricated or have features as described in one ormore of the above-incorporated U.S. Patent Applications, provided thatthe individual micro-mirrors therein are capable of undergoingpiston-like translations that do not change the angular orientations ofthe micro-mirrors.

The digital data processor 18 produces control signal sets for operatingthe MEMS-actuators that control the micro-mirrors 34 of thereconfigurable mirror array 16. That is, each MEMS actuator controls thenormal distance of the corresponding micro-mirror 34 from the referenceplane 36 in a manner responsive to a control signal set from the digitaldata processor 18. The digital data processor 18 produces one controlsignal set for each received pixel-by-pixel map of the phase andpossibly amplitude of an electric or magnetic field for an outgoingcoherent light beam 44 over a selected optical aperture. The selectedoptical aperture may be a flat surface just in front of thereconfigurable mirror array 16 or may be a cross section (CS) of theoutgoing coherent beam 44 that is both located close to thereconfigurable mirror array 16 and at a distance large compared tomaximum diameters of individual micro-mirrors 34. Typically, eachdifferent control signal set causes the MEMS actuators to set thedistances of the micro-mirrors 34 from the reference plane 36 to have adifferent non-uniform spatial distribution. The digital data processor18 typically outputs one control signal set for positioning themicro-mirrors 34 for each single-color image to be projected on theviewing screen 12. The control signal set and corresponding spatialdistribution of micro-mirror positions for an image of a first colorwill often be different from the control signal set and correspondingspatial distribution of micro-mirror positions for an image of a secondcolor even when the two images project the same shapes on the viewingscreen due to diffraction as explained below.

The digital data storage device 20 may store image data sets that thedigital data processor 18 uses to determine the control signal sets. Theimage data sets may include, e.g., pixel-by-pixel maps of relativephases over the selected aperture or pixel-by-pixel maps of bothrelative amplitudes and relative phases over the selected aperture. Thedata sets may also include control voltages for spatially positioningthe micro-mirrors 34 so as to produce the outgoing light beam 44 withsuch a pixel-by-pixel map on the selected optical aperture from thecoherent light beam 26. Thus, the reconfigurable mirror array 15functions like a hologram that reconstructs a desired spatial phasedistribution of outgoing light beam over a smooth laterally boundedsurface that is located in front of the reconfigurable mirror array 16.The data sets are communicated to the digital data processor 18 via acommunication line or bus 38.

The apparatus 10, 10′ may also include optional optical elements 40, 42for processing the outgoing coherent light beam 44 reflected off thereconfigurable mirror array 16. The optical element 40 is a refractivelens system that may adjust the divergence of the outgoing coherentlight beam 44, i.e., providing magnification. The element 42 is anoptical aperture stop, which may, e.g., filter out light that has beendiffracted by the reconfigurable mirror array 16 into higher diffractionorders.

C) Magnification of Projected 2D Image

In the apparatus 10, 10′ of FIGS. 2 and 3, the micro-mirrors 34 may haveeffective lateral linear dimensions that are small enough to causesubstantial diffraction of incident light at a wavelength, λ, outputfrom the light source 14. For example, the effective maximum diametersof the individual micro-mirrors 34 are typically larger than λ andsmaller than one of 40λ, 20λ, 10λ, and 5λ. Herein, a maximum diameter ofa micro-mirror is a diameter of the smallest circle into which thereflective surface of the micro-mirror will fit. Herein, an effectivelinear dimension, e.g., an effective maximum diameter, is the actuallateral linear dimension times any magnification that the lateral lineardimension would have when viewed through the output optics of theapparatus 10, 10′, e.g., when viewed through the lens system 40 of FIG.3.

Diffraction by the individual micro-mirrors 34 causes light that isredirected by the reconfigurable mirror array 16 to mix and interfere onthe viewing screen 12. The lateral size of the image on the viewingscreen 12 is determined by the diffraction produced by the individualmicro-mirrors 34. In particular, such diffraction enables a desirablysmall reconfigurable mirror array 16 to produce a suitably largeprojected image on the viewing screen 12.

D) Method for Projecting Images on a Viewing Screen

FIG. 5 illustrates a method 50 for projecting 2D images on a planar orflat viewing screen, e.g., projected on the non-specularly reflectingviewing screen 12 with the holographic apparatus 10, 10′ of FIGS. 2 and3.

The method 50 involves approximately reconstructing the wave front of acoherent light beam that would be able to project a desired image on theviewing screen, e.g., the viewing screen 12 of FIGS. 2 and 3. Forexample, the coherent light beam with the wave front to be reconstructedmay be a beam formed by scattering a coherent light beam by the desiredscene and/or be a beam formed by transmitting a coherent light beamthrough the desired scene. In the method 50, the wave front will bereconstructed over a selected optical aperture through which thecoherent light beam would have passed in traveling to the viewingscreen. The selected optical aperture is a finite planar region whoseform may vary in different embodiments. The selected optical aperturemay the planar region formed by the average surface of thereconfigurable mirror array 16 of FIG. 2 or 3 or may be a cross section(CS) of the outgoing light beam 44 of FIG. 2 or 3 near thereconfigurable mirror array 16.

The method 50 includes providing a pixel-by-pixel map describing theoptical field over the selected optical aperture for an actual coherentlight beam capable of projecting the desired image on a viewing screen(step 52). The pixel-by-pixel map may provide relative phases of theelectric or magnetic field of the coherent light beam over a set ofpixels that covers the selected optical aperture, e.g., the pixels maycorrespond to the individual micro-mirrors 34 of the reconfigurablemirror array 34 of FIGS. 2 and 3. The pixel-by-pixel map may optionallyprovide relative amplitudes of the electric or magnetic field of thewave front over the same set of pixels. In that case each pixel maycorrespond to a disjoint local group of micro-mirrors 34, which iscapable of approximately setting an average relative phase and relativeamplitude on the pixel.

The pixel-by-pixel map is a list whose entries are in correspondencewith the individual pixels of a pixel set that covers the selectedaperture. In the list, each entry gives an average relative phase andoptionally provides an average relative amplitude of the electric ormagnetic field of the reflected light beam at the corresponding pixel.As a first example, the pixel-by-pixel map may provide the relativephase of the portion of the coherent optical beam 26 that is reflectedby each micro-mirror 34 of the reconfigurable mirror array 16 of FIGS. 2and 3. As a second example, the pixel-by-pixel map may provide therelative phase and relative amplitude of the portion of the coherentoptical beam 26 that is reflected by each local disjoint group ofmicro-mirrors 34 in the reconfigurable mirror array 16 of FIGS. 2 and 3.The entire pixel-by-pixel map may be stored in a digital data storagedevice, e.g., the digital data storage device 20 of FIGS. 2 and 3, ormay be evaluated, e.g., by the digital data processor 18, from data forthe 2D image that are themselves stored in the same digital data storagedevice 20.

The method 50 includes configuring the reconfigurable mirror array bytranslating some of the mirrors therein such that distances of themirrors of the reconfigurable mirror array from a reference plane have anon-uniform spatial distribution (step 54). The configuring involvespositioning the micro-mirrors of the reconfigurable mirror array toreflect an incident coherent light beam in a manner that produces areflected light beam with approximately the pixel-by-pixel map providedat step 52 over the selected aperture. The incident coherent light beammay be, e.g., the light beam 26 produced by the light source 14 of FIGS.2 and 3. Then, the configuring step 54 varies, e.g., distances of someof the micro-mirrors 34 from the reference plane 36 that is parallel tothe surface of the reconfigurable mirror array 16 of FIGS. 2 and 3.

The method 50 includes then, illuminating the reconfigurable mirrorarray as configures at the step 54 with a coherent light beam such thatpart of the light beam is reflected off the array to project the desiredimage on the viewing screen, e.g., the viewing screen 12 of FIGS. 2 and3 (step 56). The illuminating coherent light beam may be typicallyspatially coherent over the height and width of the reconfigurablemirror array. Due to the positioning of the micro-mirrors at the step54, the reflected part of the illumination coherent light beam will haveat each pixel of the selected aperture about the phase of thepixel-by-pixel map provided at the step 52. For the example of thereconfigurable mirror array 16, the coherent light beam 26 would bereflected to produce the outgoing coherent light beam 44. Then, at eachpixel of the selected optical aperture, the average relative phase ofthe electric or magnetic field of the outgoing coherent light beam 44would be about that of the pixel-by-pixel map provided at the step 52.At each pixel, the phase and possibly amplitude of the outgoing coherentlight beam would be approximately those of an actual coherent light beamthat is capable of projecting the desired image on the viewing screen12. In this example, the selected optical aperture may be the averagesurface of the reconfigurable mirror array 16 or a cross section, CS, ofthe outgoing coherent light beam 44 that is close to the reconfigurablemirror array 16 but much far there from compared to the size of themicro-mirrors 34 therein. For a desired image, there may be multipleconfigurations of the micro-mirrors 34 of the reconfigurable mirrorarray 34 that are able to approximately reconstruct such an outgoinglight beam 44 that would projecting substantially the same desired imageon the viewing screen 121.

In some embodiments of apparatus 10, 10′ of FIGS. 2 and 3, performanceof the step 56 of the method 50 includes causing small vibrations of themicro-mirrors 34 of the reconfigurable mirror array 14 at a frequency ofgreater than 30 Hertz. The small vibrations cause small fluctuations oflocal coherent light intensities on the viewing screen 12 and arecontrolled to temporally wash out of interference-induced light specklein the projected image. The vibrations may be produced by vibrating theentire substrate 35 of FIGS. 2 and 3 or by vibrating individualmicro-mirrors 34 through the MEMS actuators during the performance ofthe illumination step 56. The speckle in the projected image may bereduced, e.g., using by method(s) and/or system(s) described in theabove-incorporated U.S. patent application entitled “SPECKLE REDUCTIONIN LASER-PROJECTION IMAGES”, by Vladimir A. Aksyuk, Randy C. Giles, OmarD. Lopez, and Roland Ryf.

The method 50 may include looping back to repeat the steps 52, 54, and56 in response to a control signal indicating that another desired imageis to be projected on the viewing screen, e.g., to produce a temporalsequence of 2D images on the viewing screen 12 of FIGS. 2 and 3. In someembodiments, the looping back may reconstruct new 2D images at a rate ofat least twenty (20) frames per second and preferably at a rate ofthirty (30) or more frames per second. That is, the repetition of framerate may be high enough to produce the human viewers a perception ofreal smooth video.

E) Projecting Multi-Color Images

Some embodiments of the apparatus 10, 10′ of FIGS. 2 and 3 may projectimages that a human viewer, V, would perceive as being multi-coloredimages on the viewing screen 12. One example of a suitable multicolorembodiment 14′ of the light source 14 is illustrated in FIG. 6.

Referring to FIG. 6, the multi-color light source 14′ includes, e.g.,three coherent color light sources 22R, 22G, 22B and an opticalwavelength multiplexer 62. Each of the three coherent light sources 22R,22G, 22B includes a semiconductor laser LR, LG, LB this configured tooutput light of a different color, e.g., red, green, and blue. In someembodiments (not shown), one or more of the monochromatic semiconductorlasers LR, LG, LB may be replaced by multiple lasers of the same color,e.g., to reduce speckle on viewing screen 12. Each of the coherent lightsources 22R, 22G, 22B also includes an optical switch OS that enablesindividual switching ON and OFF of the corresponding semiconductor laserLR, LG, LB. The optical wavelength multiplexer 62 redirects light fromthe three coherent light sources 22R, 22G, 22B to the expansion optics24. The optical wavelength multiplexer 62 includes, e.g., a suitablyoriented optical grating 64 as illustrated or another optical wavelengthrouting device (not shown).

The light source 14, as shown in FIGS. 2 and 3, may also include one ormore semiconductor lasers and/or multi-color laser light sources asdescribed in the above-incorporated U.S. patent application entitled“COLOR MIXTURE ILLUMINATION SOURCE AND COLOR CONTROL DATA SYSTEM” byGang Chen, Ronen Rapaport, and Michael Schabel.

Even though the multi-color light source 14′ provides the ability toproject multi-colored images, the fact that each image requiresmodulation of the produced light beam 26 via the reconfigurable mirrorarray 16 presents a complication. In particular, the method for imageformation uses the same reconfigurable mirror array 16 to modulate thelight beam 26 for different colors of light. Such an arrangement may becomplex to maintain when the image is, e.g., an object whose color is amixture of the individual colors produced by different ones of thecoherent light sources 22R, 22G, 22B as illustrated in FIG. 7.

FIG. 7 schematically illustrates an attempt to project an image of asquare on the viewing screen 12 of FIG. 2 or 3 with a singleconfiguration the reconfigurable mirror array 16 when the square has acolor that is a mixture of colors of two the semiconductor lasers LR, LGof FIG. 6. Since the reconfigurable mirror array 16 typically producessome diffraction in the outgoing coherent light beam 44, the singleconfiguration of the reconfigurable mirror array 16 will typicallyproject a first image I1 with the light from the semiconductor lasers LRand will project a different second image 12 with light from the othersemiconductor laser LG. That is, the first and second images I1, I2 willnot completely coincide on the viewing screen 12 due to the differentamounts of diffraction produced on light of different colors. Thisnon-coincidence problem can be resolved by combining time division colormultiplexing with the use of spatial configurations of thereconfigurable mirror array 16 that depend on the color of the laserlight being projected on the viewing screen 12.

FIG. 8 illustrates a method 70 for producing multi-color lightprojection, i.e., an image, on a viewing screen, e.g., the viewingscreen 12 of FIG. 2 or 3. The method may use the apparatus 10, 10′ ofFIG. 2 or 3 with the multi-color light source 22′ of FIG. 6. Themulti-color light projection is a superposition of one or more images inthe colors of single-color light sources, e.g., the single-color lightsources 22R, 22G, 22B of FIG. 6.

The method 70 includes selecting a single color image in a superpositionof such images that forms the desired multi-color light projection (step72).

The method 70 includes performing steps 52, 54, and 56 of the method 50to project the selected single color image on the viewing screen withlight from the corresponding single color light source (74). Theperforming of step 52 includes providing a pixel-by-pixel map of a phaseof the optical field for light of the selected single color over theselected optical aperture. In some embodiments, the providingpixel-by-pixel map will also provide the amplitude of the optical fieldfor the light of the selected single color over the selected opticalaperture.

The method 70 includes looping back (76) to repeat steps 72 and 74 foreach remaining single color image in the superposition that gives thedesired multi-color projection, i.e., to perform time division colormultiplexing (TDCM). In each repetition of the steps 72 and 74, theprovided pixel-by-pixel map provided at substep 52 of step 74 is basedon the single color image to be projected at that repetition. Inparticular, the repetition would typically involve providing a newpixel-by-pixel map for each with a different single color image of thesuperposition. Such a repetition would also typically includereconfiguring the reconfigurable mirror array by translating some of themirrors therein such that the distances of the mirrors of from thereference plane have a new non-uniform spatial distributioncorresponding to the new pixel-by-pixel map. Finally, each repetitionwith a different single color image would typically include illuminatingthe reconfigured array with a coherent light beam of the different colorsuch part of the coherent light beam of the different color is projectedon the planar viewing screen.

The method 70 includes repeating steps 72, 74, and 76 at a frequencygreater than the frequency needed for the human perception of smoothvideo (step 78). The repetition of the steps 72, 74, and 76 is needed toproduce the perceived multi-colored projection based on time divisionmultiplexing of single color images. In particular, each of the singlecolor images of the superposition for the multi-colored projection wouldtypically be projected for a time shorter than the inverse of thefrequency for the human perception of smooth movement. Thus, a humanviewer would perceive a single multi-colored image that averages thesingle color images rather than a sequence of single color images.

To produce such a “perceived” multi-color projection, the light source14′ would be controlled to perform time division color multiplexing at ahigh enough rate for the perception of a constant multi-colored image toa human viewer, V. In particular, the loop back or repeat frequency inthe method 70 would be, at least, 20 Hertz and preferably would be 30Hertz or more to give such a perception to a human viewer.

In some embodiments, the time division color multiplexing may beperformed so that the single color images of different color areprojected on the viewing screen for different lengths of time. Suchunequal weighting of per-color projection times would then, vary theperceived intensities of the various single colors in the finalsuperposition. Projecting a single color image for a larger part of theviewing time should cause a viewer to perceive that that color morestrongly than would otherwise be the case.

F) Ways of Positioning Micro-Mirrors in Holographic Image Projector

Referring to FIGS. 2 and 3, there are various ways for fixing thepositions of the micro-mirrors 34 such that the reconfigurable mirrorarray 16 produces from the incident coherent light beam 26 an outgoingcoherent light beam 44 with a desired pixel-by-pixel map on the selectedoptical aperture. Some embodiments may fix the positions of the wholeset of micro-mirrors 34 together to directly so that the light reflectedby the reconfigurable mirror array 16 satisfies the map over theselected optical aperture. Other embodiments may separately determinethe relative amplitude and relative phase of the light reflected byseparate and disjoint local operating units in the reconfigurable mirrorarray 16. Each local operating unit includes, e.g., 2, 3, 4, or moreneighboring micro-mirrors 34. In a local operating unit, an averagedistance of the micro-mirrors 34 from the reference plane 36 is adjustedso that light reflected by the local operating unit has an averagerelative phase satisfying the provided map at a corresponding pixel anddifferences between distances of the micro-mirrors 34 therein from thereference plane 36 are adjusted so that the average amplitude of lightreflected by the local operating unit to the same pixel satisfies theprovided map. This later approach to setting the configurations ofgroups of micro-mirrors is described for example in U.S. patentapplication Ser. No. 11\448,390, filed by Girsh Blumberg on Jun. 6, 2006(Herein, referred to as the '390 application.). This U.S. patentapplication is incorporated herein by reference in its entirety. In eachof the above approaches, configuring the reconfigurable mirror array 16involves setting the distance of each micro-mirror 34 from the referenceplane 36.

G) Evaluating a Pixel-by-Pixel Map of a Wave Front Over an Aperture

In methods 50 and 70 of FIGS. 5 and 8, several methods are available forproducing pixel-by-pixel maps that enable configuring the micro-mirrorsof a reconfigurable mirror array to modulate an incident coherent lightbeam in a manner that would project a desired image on the viewingscreen. The processor 18 of FIG. 2 or 3 may perform, e.g., one of themethods to determine positions for the micro-mirrors 34 that wouldenable the reconfigurable mirror array 16 to project the desired imageonto the viewing screen 12.

In the first and second methods, the light wavefront g(m1, m2) just infront of the reconfigurable mirror array is a discrete inverse Fouriertransform (IFT) of the image light wavefront f(n1, n2) at the viewingscreen. Here, the 2D vectors (m1, m2) index the micro-mirrors (pixels)of the reconfigurable array of micro-mirrors, and the 2D vectors (n1,n2) index the pixels on the viewing screen. For these methods, the imageis treated as being in the far field region for the reconfigurablemirror array.

In a first and second methods, the processor 18 of FIGS. 2 and 3performs iterative algorithms to generate suitable spatialpixel-by-pixel maps of the relative phase of incident coherent lightreflected off each mirror of the reconfigurable mirror array. Theproduced pixel-by-pixel maps are pure phase modulations of the lightwavefront incident on the reconfigurable mirror array. Such maps may begenerated by versions of the Gerchberg-Saxton algorithm. Some versionsof the Gerchberg-Saxton algorithm are described, e.g., in one or more ofthe articles published at Optics Letters, Vol. 21, No. 12 (Jun. 15,1996) pages 842-844; Optics Letters, Vol. 3, No. 1 (July 1978) pages27-29; Applied Optics, Vol. 21, No. 15 (Aug. 1, 1982) pages 2758-2769.These three articles are incorporated herein by reference in itsentirety.

FIG. 9 illustrates a first iterative method 80 for determining anappropriate pixel-by-pixel map, e.g., an appropriate g(m1, m2) at thereconfigurable mirror array. The first iterative method 80 is based onthe spatial distribution of monochromatic image light intensities, i.e.,f·f*(n1, n2), on the viewing screen.

The first iterative method 80 includes determining the spatialdistribution of absolute light amplitudes at the viewing screen based onthe desired image thereon, i.e., determining |f(n1, n2)| (step 82). Theabsolute amplitudes are absolute values of the amplitude of the imagelight wavefront at the individual pixels, i.e., the (n1, n2)s, of theviewing screen.

Next, the first iterative method 80 includes forming a new spatialdistribution of complex light amplitudes, i.e., h(n1, n2), for the imageby multiplying each of the absolute light amplitudes by a phase (step84). If the multiplying phase at pixel (n1, n2) is given bye^(iφ(n1,n2)), the new spatial distribution of complex light amplitudesis defined by:

h(n1,n2)=e ^(iφ(n1,n2)) ·|f(n1,n2)|.

For any image, the phases may be selected in a variety of ways, becausethe perceived form of an image depends on the magnitudes of lightamplitudes at the viewing screen and does not depend on the phases ofthe light amplitudes at the viewing screen. For example, the phases maybe selected by any pseudo-random selection processes.

Next, the first iterative method 80 includes performing an iterativeprocess to produce after N iterations a new discrete IFT functiong_(N)(m1, m2) that can produced by performing a pure phase modulation ofan incident coherent light beam with the reconfigurable mirror array(step 86). For example, the apparatus 10′ of FIG. 3 may produce adiscrete IFT function g_(N)(m1, m2) with a substantially constantmagnitude over the micro-mirrors of the reconfigurable mirror array. Thenew g_(N)(m1, m2) has a discrete FT h_(N+1)(n1, n2) that gives thedesired image on the viewing screen.

At the k-th cycle, the iterative process involves performing severalsubsteps. First, the iterative process involves obtaining a discreteIFT, i.e., g_(k)(m1, m2), of the last spatial distribution of compleximage light amplitude, i.e., h_(k)(n1, n2) (substep A). At the firstiteration, the spatial distribution h₁(n1, n2) is the spatialdistribution h(n1, n2) that was obtained at above step 84. Next, theiterative process includes replacing the discrete IFT g_(k)(m1, m2) by anew spatial distribution of pure phases, i.e., g_(k)′(m1, m2) (substepB). The distribution g_(k)′(m1, m2) is given by:

g _(k)′(m1,m2)=g _(k)(m1,m2)/|g _(k)(m1,m2)|.

Next, the iterative process involves taking a discrete FT of the spatialdistribution of pure phases g_(k)′(m1, m2) to obtain a new spatial imagedistribution f_(k)(n1, n2) on the viewing screen (substep C). Next, theiterative process involves determining whether the g_(k)′(m1, m2) hassufficiently converged for projection of a suitable image on the viewingscreen (substep D). Some embodiments may determine that sufficientconvergence has occurred in response to performance of a preselectednumber of iterations, i.e., iterations of the loop formed by substeps A,B, C, D, and F. For example, the preselected number may be 5 iterationsfor low quality images, 10 iterations for a medium quality images, and100 iterations for high quality images. Of course, the preselectednumbers of iterations may vary for different embodiments. Alternateembodiments may involve evaluating an error, E, whose value is then,used to determine the number of iterations at which sufficientconvergence occurs. In one such embodiment, the error E is a sum overthe viewing screen pixels, i.e., (n1, n2), and is defined as:

E=Σ _(image pixels(n1,n2)) [|f(n1,n2)|² −|FT{g _(k) /|g_(k)|}(n1,n2)|²]².

In such embodiments, the iterative process is sufficiently convergedwhen the discrete FT of the phase pattern on the reconfigurable mirrorarray provides an image that is sufficiently close to the desired image.In another embodiment, the above equation for the error E is modified byreplacing g_(k)/|g_(k)| by g_(k) on the right-hand side thereof. Such amodification is available, because g_(k)(m1, m2) typically converges toa distribution of pure phases after an adequate number of iterations.Alternately, the error E may be a sum over the pixels or micro-mirrorsof the reconfigurable mirror array. One such error E is given by:

E=Σ _((m1, m2) pixels of array) [[g _(k)(m1, m2)]² −[g _(k)′(m1,m2)]²]².

In such embodiments, the convergence is sufficient when the functiong_(k)(m1, m2) is sufficiently precisely represented by a spatialdistribution of pure phases on the reconfigurable mirror array. If theiterative process is determined to have sufficiently converged atsubstep D, the iterative process then, includes vertical positioning theindividual micro-mirrors of the reconfigurable mirror array in a mannerthat produces the g_(k)′(m1, m2) phase modulation on a light beamincident on the reconfigurable mirror array (step E). That is, themicro-mirror at lateral (m1, m2) is positioned to produce a wavefrontwhose local phase is g_(k)′(m1, m2)/|g_(k)′(m1, m2)| from the portion ofthe coherent light beam that will be incident on that micro-mirror. Ifthe iterative process has not yet sufficiently converged, the iterativeprocess includes evaluating the next spatial distribution of compleximage light amplitudes, i.e., h_(k+1)(n1, n2), at the viewing screen(substep F). The next spatial distribution h_(k+1)(n1, n2) is defined asfollows:

h _(k+1)(n1,n2)=|f(n1,n2)|[f _(k)(n1,n2)/|f _(k)(n1,n2)|].

Here, f_(k)(n1, n2) is the discrete FT at the viewing screen of thespatial distribution g_(k)′(m1, m2) on the reconfigurable mirror array.Thus, the phases and magnitudes of the next image light wavefronth_(k+1)(n1, n2) are defined by the discrete FT of g_(k)′(m1, m2) and thedesired image, respectfully. After substep F, the iterative processincludes repeating substeps A-D and E or F for the (k+1)-th iteration.

FIG. 10 illustrates the second iterative method 80′ for finding anappropriate pixel-by-pixel map, e.g., a g(m1, m2) at the reconfigurablemirror array 16 that would cause the projection of the desired image onthe viewing screen 12. The second iterative method 80′ is also based onthe spatial distribution of monochromatic image light intensities, i.e.,f·f*(n1, n2), on the viewing screen.

The second iterative method 80′ includes repeating steps 82, 84, and 86as described with respect to the method 80 of FIG. 9, but some substepsof the step 86 are modified in the second iterative method 80′, i.e.,due to a new iterative process.

At the k-th cycle, the new iterative process associated involvesperforming already described substeps A-F as described for step 86 withthe following modifications. At substep B, the iterative processinvolves replacing the discrete IFT g_(k)(m1, m2) by a new spatialdistribution g_(k)′(m1, m2), which is given by:

g _(k)′(m1,m2)=g _(k)(m1,m2)+λ[g _(k)(m1,m2)/|g _(k)(m1,m2)|−g_(k)(m1,m2)].

Thus, the new spatial distribution has a memory of the magnitudes of thevalues in the discrete IFT. At substep F, the new iterative processincludes defining the next distribution of complex light amplitudes overthe viewing screen as follows:

h _(k+1)(n1,n2)=h _(k)(n1,n2)+λ[|f(n1,n2)|[f _(k)(n1,n2)/|f_(k)(n1,n2)|−h _(k)(n1,n2)].

Thus, the new iterative process has a memory of the magnitudes of thevalues in the discrete FT. In the new iterative process, the realpositive parameter λ have, e.g., a small positive value, λ≅1.5. Theselection of the λ.may affect the rapidity of the convergence of theiterative process.

In a third method, a pixel-by-pixel map of both the relative amplitudeand the relative phase is obtained for a light beam capable ofprojecting the desired image on a viewing screen via two measurements.In the first measurement, the scene-to-be-imaged scatters or transmits acoherent light beam, and the intensity of the scattered or transmittedlight is measured over a selected aperture. In the second measurement, acoherent light beam is again transmitted or scattered by thescene-to-be-imaged and is then, interfered with a known coherentreference beam. Then, the intensity of the interfered beams is measuredover the selected aperture to obtain a pixel-by-pixel map of the phaseof the transmitted or scattered light beam over the selected aperture.This method for obtaining a pixel-by-pixel map of both amplitude andphase information over a selected optical aperture is described forexample in the above-mentioned '390 application.

Other techniques for finding a pixel-by-pixel map of relative phasesand/or amplitudes over a selected optical aperture for a light beam thatwould project a desired image on a viewing screen may be described inone or more of “Diffraction-Specific Fringe Computation forElectro-Holography”, Ph.D. Thesis of Mark Lucente in the Department ofElectrical Engineering and Computer Science, Massachusetts Institute ofTechnology, September 1994; U.S. Pat. No. 6,211,848; U.S. Pat. No.4,834,476; U.S. Pat. No. 4,986,619; and U.S. Pat. No. 5,172,251. Theabove-listed thesis U.S. patents and U.S. patent application areincorporated herein by reference in their entirety.

H) Direct Image Projection with a Reconfigurable Mirror Array

FIGS. 11A-11E illustrate other apparatus 10A, 10B, 10C, 10D, 10E forprojecting images, i.e., perceived as single colored or multi-colored,on viewing screen 12. The apparatus 10A-10E include 2D reconfigurablespatial light modulator 16, 16′, coherent light source 14, digital dataprocessor 18, digital data bus 38, and digital data storage device 20.Each of these elements of the apparatus 1A-10E may function, beinter-arranged and/or be constructed substantially similarly or the sameas in same-numbered elements of the apparatus 10, 10′ of FIGS. 2-3,i.e., except as distinguished below.

Herein, a reconfigurable spatial light modulator refers to either a 2Dreconfigurable mirror array, e.g., the array 16 of micro-mirrors 34, asshown in FIGS. 11A-11D, or a 2D reconfigurable liquid crystal array,e.g., the array 16′ of liquid crystal cells 34′, as shown in FIG. 11E. Areconfigurable spatial light is able to modulator phase and/or amplitudemodulate an incident coherent light wavefront in a spatially non-uniformand reconfigurable manner.

In FIGS. 11A-11D, the 2D reconfigurable mirror array 16 may have anyconstruction already described with respect to the apparatus 10, 10′ ofFIGS. 2-3. That is, the reconfigurable mirror array 16 of the apparatus10A-10D has, at least, some micro-mirrors 16 that perform piston-likemotions under the control of MEMS actuators, e.g., motions perpendicularto the imaginary plane 36. The MEMS actuators are located in/on theplanar substrate 35.

In FIG. 11E, the 2D reconfigurable liquid crystal array 16′ includes aregular 2D array 89 of substantially identical liquid crystal cells 34′and a linear polarizer 90, 90′ located on each side of the regular 2Darray 89. Each liquid crystal cell 34′ holds a birefringent liquidcrystal and has electrodes adjacent on one or both surfaces thereof. Theelectrodes are configured to apply voltages that rotate the optical axisof the liquid crystal of the same liquid crystal cell 34′. Theelectrodes are controlled so that different liquid crystal cells 34′ canbe independently addressed. That is, the optical axes of different onesof the liquid crystal cells 34′ can be independent rotated, under thecontrol of the digital data processor 18. The linear polarizers 90, 90′may be configured to have their polarization directions in a variety ofrelative orientations, e.g., parallel or perpendicular. The sandwich ofa liquid crystal cell 34′ and polarizers 90, 90′ may produce on anincident coherent light wavefront a phase modulation or a combination ofphase and amplitude modulation. Thus, the reconfigurable liquid crystalarray 16′ can cause a controllable spatially non-uniform modulation ofan incident light wavefront. The apparatus 10E modulates an incidentillumination light beam 26 transmitted through the 2D reconfigurableliquid crystal array 16′. Other embodiments (not shown) may include a 2Dreconfigurable liquid crystal array that modulates and incident lightbeam reflected from the reconfigurable liquid crystal array. Indeed,other embodiments image projection apparatus may setups similar to theapparatus 10A-10D of FIGS. 11A-11D except that the reconfigurable mirrorarray 16 is replaced by a reconfigurable reflective liquid crystalarray.

In contrast to the apparatus 10, 10′ of FIGS. 2-3, the apparatus 10A-10Eof FIGS. 11A-11E use a direct one-to-one correspondence between thepixels of reconfigurable spatial light modulator 16, 16′ and the pixelsof the desired image. From the one-to-one correspondence, each pixel ofthe reconfigurable spatial light modulator 16, 16′ in FIGS. 10A-10E maybe configured based on the desired light intensity of the singlecorresponding pixel of the desired image. Thus, regions of thereconfigurable spatial light modulator 16, 16′ are mapped directly intocorresponding regions of the image to be projected onto the viewingscreen 12. For that reason, the digital data processor 18 of manyembodiments of the apparatus 10A-10E will not perform a complexiterative process over the entire reconfigurable mirror array 16together to determine how to position an individual micro-mirror 34 orto configure an individual liquid crystal cell 34′ so that thereconfigurable spatial light modulator 16, 16′ projects a desired image.For example, the digital data processor 18 does not evaluate an inverseFourier transform of the desired image in order for the apparatus10A-10E to project the desired image on the viewing screen 12. For theabove reasons, the digital data processor 18 of some embodiments of theapparatus 10A-10E may more rapidly determine positions of themicro-mirrors 16 or configurations of the liquid crystal cells 16′ for aspecific image than some embodiments of the apparatus 10, 10′ of FIGS.2-3.

The apparatus 10A-10E include additional functional elements that maynot be present in embodiments of the apparatus 10, 10′ of FIGS. 2-3. Theadditional elements include first focusing optical lens system 92,second focusing optical system 94, and optical transmission filter 96.

The first focusing lens system 92 is located on the portion of theoptical path that is either after the reconfigurable spatial lightmodulator 16, 16′, i.e., as in the apparatus 10A-10B and 10E, before thereconfigurable spatial light modulator 16, 16′, i.e., as in theapparatus 10C, or both before and after the reconfigurable spatial lightmodulator 16, 16′, as in the apparatus 10D. In various applications, thefirst focusing lens system 92 may be located in different ones of theabove-listed positions to satisfy space/geometry constraints, e.g.,constraints for integrating the one of the image projection apparatus10A-10E into a small cell phone.

The transmission filter 96 is located at or near a plane 98 in which thefirst focusing filter 92 forms an image of the illumination light beam26 incident onto the reconfigurable spatial light modulator 16, 16′. Forexample, if the illumination light beam 26 is substantially collimated,e.g., as shown in FIGS. 11A-11B and 11E, the plane 98 is at or near thefocal plane of the first focusing lens system 92. In contrast, if theillumination light beam 26 is converging, e.g., as illustrated in FIGS.11C and 11D, the plane 98 is located closer to the first focusing lenssystem 92 than the focal length. In the plane 98, the first focusinglens system 92 forms, e.g., a Fourier transform of the phase-modulatedwavefront outgoing from the reconfigurable spatial light modulator 16,16′.

At the plane 98, the first focusing lens system 92 causes the outgoinglight beam 44 to form a light pattern, e.g., as illustrated in FIGS.12A-12B. The light pattern has bright spots 110, 112, 114 that groupinto different diffractive orders, e.g., due to the approximate 2Dperiodicity of the lateral distribution of micro-mirrors 34 or liquidcrystal cells 34′ in the reconfigurable spatial light modulator 16, 16′.The transmission filter 96 transmits to the rest of the image projectionapparatus 10A-10E light from substantially only one of the bright spots110, 112, 114 or from one diffractive order in the pattern of brightspots 110, 112, 114. The transmission filter 96 substantially blockslight from the other bright spots 110, 112, 114 of the light pattern onor near the plane 98. The light from the single unblocked bright spot110, 112, 114 or unblocked diffractive order passes through theremaining lens system(s) 94, 100 to form the image on the viewing screen12.

The transmission filter 96 provides an attenuation that varies with thelateral position on the plane 98. The attenuation may vary smoothly withposition, R, on the plane 98. For example, the attenuation may depend onthe position, R, as A·exp[−([R−R_(o)]/w)²/2] where R_(o) and 2w are therespective center and width of the transmission filter 96. At placeswhere |R−R_(o)|=2w, the transmission filter 96 attenuates incident lightintensities by e⁻² more than it attenuates the incident light intensityat its center.

FIGS. 12A and 12B illustrate how R−R_(o)|=2w boundaries of theabove-described exemplary transmission filter 96 might be located in theplane 98. These boundaries are indicated by dashed circles 116, 118 inFIGS. 12A-12B. In the example of FIG. 12A, the center of thetransmission filter 96 is at the center of the bright spot 110, i.e.,the zeroth diffractive order bright spot. In this embodiment, thetransmission filter 96 substantially blocks the light from the otherbright spots 112, 114 of the other diffractive orders. In the example ofFIG. 12B, the center of the transmission filter 96 is at the center ofone of the bright spots 112 of the first diffractive order. In thisembodiment, the transmission filter 96 substantially blocks light fromthe other diffractive orders and from other bright spots 112 of thefirst diffractive order. The transmission filter 96 could also beconfigured to substantially transmit all of the bright spots 112 of thefirst diffractive order.

The second focusing lens system 94 is located on the portion of theoptical path behind the transmission filter 96. In particular, thetransmission filter 96 may be located in the focal plane of the secondfocusing lens system 94, i.e., as illustrated in FIGS. 11A-11B and 11E,or may be located much closer than the focal length f′ from the secondfocusing system 94, as illustrated in FIGS. 11C-11D. The second focusinglens system 94 produces a Fourier transform or an inverse Fouriertransform of the light wavefront transmitted by the transmission filter96 at a late plane, e.g., at the viewing plane 12.

By serially interleaving the Fourier transforms performed by thefocusing lens systems 92, 94 with the spatial filtering performed by thetransmission filter 96, direct imaging of individual pixels of thereconfigurable spatial light modulator 16, 16′ to individual pixels ofthe viewing screen seems substantially possible. The combined imagingprocess also enables the phase-modulated wavefront at the reconfigurablespatial light modulator 16, 16′ to be converted into a visible wavefronton the viewing screen 12.

Some embodiments of the apparatus 10A-10E may include a lens system 100,e.g., as shown in FIGS. 11A-11B and 11E. The lens system 100 magnifiesthe lateral dimensions in the image being projected onto the diffusivelyreflective surface of the viewing screen 12.

Some embodiments of the apparatus 10A-10E project images, which areperceived as being multi-colored. In such embodiments, the coherentlight source 14 may be the time-interleaved multi-color laser lightsource 14′ of FIG. 6. In such embodiments, the reconfigurable spatiallight modulator 16, 16′ may be reconfigured at the color-interleavingrate to project different images for each interleaved color.

The micro-mirrors 34 and liquid crystal cells 34′ of the apparatus10A-10E may be operated individually or may be operated in localoperating groups (LOGs). In the later case, a disjoint set of two, threeor more neighboring micro-mirrors 34 or liquid crystal cells 34′, asappropriate, forms each local operating group. Each operating groupfunctions as a single pixel of the reconfigurable spatial lightmodulator 16, 16′. In some such embodiments, a proper subset of themicro-mirrors 34 or liquid crystal cells 34′ of each local operatinggroup are immobile and without a corresponding MEMS actuator or voltagecontroller.

The apparatus 10A of FIG. 11A may also include a glass wedge 102 that isconfigured to increase the incidence angle of the illumination lightbeam 26 on the reconfigurable mirror array 16. In some multi-coloredembodiments, the coherent light source 14 may direct the differentcolors of the illumination light beam 26 onto the glass wedge 102 and/orreconfigurable mirror array 16 at somewhat different incidence angles sothat images of different colors are projected onto the same lateral partof the viewing screen 12. In particular, such color-dependent incidenceangle variations are typically needed when a nonzero diffractive orderis selectively transmitted by the transmission filter 96. The apparatus10 of FIG. 2 may also include a glass wedge (not shown) similar to andsimilarly configured to the glass wedge 102 of FIG. 11A.

In the various embodiments of the apparatus 10A-10E, the digital dataprocessor 18, data bus 38, and digital data storage 20 control MEMSactuators that are located in or on the planar substrate 35 or controlelectrodes on the liquid crystal cells 34′. MEMS actuators control andadjust positions of some or all of the micro-mirrors 34 as describedwith respect to the apparatus 10, 10′ of FIGS. 2-3. Nevertheless, toproject the same desired image, the digital data processor 18, data bus38, and digital data storage 20 of the apparatus 10A-10D may set up adifferent spatial configuration for the micro-mirrors 34 than wouldotherwise be set up for the apparatus 10, 10′ of FIGS. 2-3.

FIG. 13 illustrates one method 120 of directly imaging a phase-modulatedwavefront on a viewing screen, e.g., with the apparatus 10A-10E shown inFIGS. 11A-11E.

The method 120 includes phase or amplitude modulating a wavefront of anincident coherent illumination light beam to produce an outgoing lightbeam having a spatially phase-modulated wavefront, e.g., the outgoinglight beam 44 (step 122). The modulating step 122 may involveeffectively multiplying a wavefront of an incident illumination lightbeam 26 by a space dependent reflection factor A·exp[iΦ(m1, m2)] or by aspace dependent transmission factor A(m1, m2)·exp[iΦ(m1, m2)] to producethe outgoing light beam 44. Here, A and A(m1, m2) are amplitudes, andΦ(m1, m2) is a phase angle. Also, the integer-component vector (m1, m2)indexes spatial positions of pixels along the wavefront. The first andsecond components of the vector (m1, m2) correspond to row and columnlocations of either a single micro-mirror 34 or liquid crystal cell 34′or a local operating group of neighboring micro-mirrors 34 or liquidcrystal cells 34′. Such a modulation may be performed by spatial lightmodulators such as the reconfigurable mirror array 16 of FIGS. 11A-11Dor the reconfigurable liquid crystal array 16′ of FIG. 11E. For the (m1,m2) micro-mirror 34 or liquid crystal cell 34′, the corresponding phaseΦ(m1, m2) and/or amplitude, i.e., A or A(m1, m2), may be evaluated basedon the desired light intensity at the correspondingly located imagepixel (m1, m2) at viewing screen 12.

The method 120 includes focusing a light beam such that the outgoinglight beam can form a diffractive light pattern on a distant plane,e.g., a plane distant from the spatial light modulator, wherein thelight pattern is approximately a Fourier transform of the modulatedwavefront produced at the modulating step 122 (step 124). The distantplane may be, e.g., the focal plane of the lens system that causes thefocusing, e.g., as shown in FIGS. 11A-11E. The focusing step 124 may beperformed on a coherent illumination light beam to produce theconverging light beam 26 incident on the spatial light modulator, e.g.,the reconfigurable mirror array 16 in the apparatus 10C-10D of FIGS.11C-11D. The focusing by the first focusing lens system 92 causes theoutgoing light beam 44 to produce a diffractive light pattern in thedistant plane 98 of FIGS. 11C-11D. The focusing step 124 mayalternatively be performed on the light beam 44 that is outgoing fromthe spatial light modulator, e.g., as in the reconfigurable mirrorarrays 16 of FIGS. 11A-11B and the reconfigurable liquid crystal array16′ of FIG. 11E. This later embodiment of the focusing step 124 alsocauses the outgoing light beam 44 to produce such a diffractive lightpattern on the distant plane 98.

The method 120 includes spatially filtering the light from thediffractive light pattern so that only one bright spot thereinsubstantially transmits light to the rest of the optical imageprojection system (step 126). In particular, the unfiltered lightpattern has different bright spots due to the approximate 2D periodicityof the arrangement of micro-mirrors 34 or liquid crystal cells 34′ inthe reconfigurable spatial light modulator 16, 16′. The spatialfiltering may involve, for example, filtering with the transmissionfilter 96 of FIGS. 11A-11E to block all but a single such diffractiveorder of the light pattern on the plane 98 from contributing to theimage to be projected onto the viewing screen 12. In particular, thespatial filtering may be configured to transmit a bright spot of thezeroth order or of a higher order in the diffractive light pattern. Ifthe spatial filtering transmits light from a higher order, light fromeither one or more than one of the bright spots of said higher order mayalso be transmitted. Indeed, transmitting light from more than onebright spot of the same order can increase the brightness of the imagethat is projected on the viewing screen 12.

The method 120 includes sending the spatially filtered light, i.e., fromthe unblocked bright spot(s) of the diffractive light pattern, throughanother focusing lens system to project a single color image onto aviewing screen (step 128). The image is typically a Fourier transform oran inverse Fourier transform of the light pattern transmitted by thespatial filtering step 126. The transmitting step 128 includes passingthe spatially filtered light beam through focusing lens systems(s) 94,100 in the apparatus 10A-10E of FIGS. 11A-11E.

Some embodiments of the method 120 include repeating the steps 122, 124,126, and 128 for coherent illumination light beams of different colors,e.g., red, green and blue. Indeed, the beam's color may be rotated inround robin fashion for the successive performances of the steps 122,124, 126, and 128 to produce color interleaving. Such color interleavingmay be performed at a high enough frequency to cause a perception that amulti-colored image is projected onto the viewing screen, e.g., theviewing screen 12 of FIGS. 11A-11E.

Various embodiments of the method 120 may implement the modulating step122 differently. Two exemplary embodiments of the modulating step 122are illustrated in FIGS. 14A and 14B. In each such embodiment, thetransmission filter 96 may be configured to substantially transmit onlythe zeroth diffractive order of the light pattern on the plane 98 asillustrated in FIG. 12A.

Below, a micro-mirror 34 (liquid crystal cell 34′) at row m1 and columnm2 of the reconfigurable mirror array 16 is referred to as an evenmicro-mirror (even liquid crystal cell) if the sum (m1+m2) is even andis referred to as an odd micro-mirror (odd liquid crystal cell) if thesum (m1+m2) is odd. Even and odd micro-mirrors 34 (liquid crystal cells)are indicated in FIG. 14A as white squares and crosshatched squares,respectfully.

FIG. 14A illustrates an embodiment in which the modulating step 122involves positioning each micro-mirror 34 or liquid crystal cell 34′separately based on the desired brightness of the corresponding pixel ofthe image-to-be-projected onto the viewing screen 12. In one such anembodiment, the micro-mirrors 34 of the reconfigurable mirror array 16would be aligned along a single plane during the modulating step 122 ifthe image-to-be-projected had a uniform nonzero brightness on theviewing screen 12. In this embodiment, the micro-mirrors 34 of thereconfigurable mirror array 16 would be maximally distant from said samesingle plane if the image-to-be-projected had a uniform brightness ofzero on the viewing screen 12. In the later case, the distribution ofthe micro-mirrors 34 with respect to said single plane would form acheckerboard pattern as shown in FIG. 14A. In the checkerboard pattern,the odd micro-mirrors 34 are positioned to one side of the single plane,and the even micro-mirrors are positioned to the other side of the samesingle plane. In other situations where the brightness varies frompixel-to-pixel over the image-to-be-projected, the modulating step 122would involve positioning each micro-mirror 34 at a distance from thesame single plane in a manner that depends on the brightness of thecorresponding pixel of the image-to-be-projected. In particular, if thedesired light intensity I at a pixel of the image is I, then, thecorresponding micro-mirror 34 of the reconfigurable mirror array 16would be positioned to produce a relative phase, Φ, on the wavefrontoutgoing from that portion of the reconfigurable mirror array 16. Here Φwould be defined as follows.

Φ=±arccos([I/I _(max)]^(1/2)).

In the above equation, the ±sign indicates the side of the same plane,and I_(max) is the maximum brightness of any pixel of theimage-to-be-projected. The sign is positive for even micro-mirrors 34and is negative for odd micro-mirrors 34, because the modulating step122 would position such even and odd micro-mirrors 34 on opposite sidesof the single plane on which they would be located for an image ofuniform nonzero brightness.

FIG. 14B illustrates an embodiment in which the modulating step 122involves positioning disjoint neighboring pairs of micro-mirrors 34 orliquid crystal cells 34′ separately, i.e., local operating groups(LOGs). In FIG. 14B, the boundaries between local operating groups, LOG,are indicated by double lines, and the boundaries the betweenmicro-mirrors 34 of one such embodiment are indicated by single lines.In each such local operating group, LOG, one micro-mirror 34 isindicated by a white square, and the other micro-mirror 34 is indicatedby a crosshatched square. The micro-mirrors 34 indicated by whitesquares and crosshatched squares are moved differently during themodulation step 122.

In some such embodiments, the micro-mirrors 34, which are indicated bycrosshatched squares are immobile, i.e., not MEMS actuated, and themicro-mirrors 34, which are indicated by white squares, are moveable byMEMS actuators. During the modulating step 122, each micro-mirror 34corresponding to a white square is positioned by its MEMS actuator basedon the brightness of the corresponding pixel of theimage-to-be-projected. In particular, each such micro-mirror 34 ispositioned to cause a relative phase change of Φ whereΦ=2·arcos([I/I_(max)]^(1/2)) on the wavefront incident thereon. That is,the mobile micro-mirror 34 of the local operating group is moved twiceas far to produce for same pixel brightness than in the embodiment whereeach pixel of the reconfigurable mirror array 16 has a singlemicro-mirror 34.

In other such embodiments, both micro-mirrors 34 of an operating groupmay be MEMS actuated and mobile. Then, in each operating group, the twomicro-mirrors 34 could be positioned to cause a relative phase change ofΦ of (2·arcos([I/I_(max)]^(1/2)), e.g., between the two micro-mirrors34. In some such embodiments, the central positions of the micro-mirrors34 of a local operating group may also be adjusted empirically to reducediffraction-caused light contamination in pixels neighboring the pixelthat corresponds to the local operating group in theimage-to-be-projected. For example, such an adjustment may involverepositioning local operating groups in the vertical and/or horizontaldirections to form a checkerboard/alternating pattern as much aspossible.

Referring again to FIGS. 11A-11D, some embodiments of apparatus 10A-10Dinclude a transmission filter 96 configured to transmit light in aselected nonzero order of the diffractive light pattern in the plane 98.In such embodiments, the micro-mirrors 34 of the reconfigurable mirrorarray 16 may be blazed, e.g., to increase the percentage of illuminationlight that is directed into the bright spot from which light isselectively transmitted by the transmission filter 96. Such blazing mayinvolve tilting the reflective surfaces of said micro-mirrors 34 at anon-normal incident angle with respect to the incident light beam 26and/or may involve etching or moving group of mirrors such to createstaircases onto the reflective surfaces of said micro-mirrors oroperating group 34 to simulate blazing.

In various embodiments of apparatus 10E of FIG. 11E, the individualliquid crystal cells 34′ or local operating groups thereof may also beconfigured to produce the relative above-described phases on theincident light wavefront. In particular, the relative phases, Φ, maydepend on light intensities at the corresponding pixels of the desiredimage according to the same formulas, e.g., Φ=arcos([I/I_(max)]^(1/2)).

From the disclosure, drawings, and claims, other embodiments of theinvention will be apparent to those skilled in the art.

1. An apparatus, comprising: a light source comprising a plurality ofcoherent sources, each coherent source being configured to emit light ofa different color than the remaining of the coherent sources; areconfigurable array of mirrors and MEMS actuators, each mirror beingcontrolled by a corresponding one of the MEMS actuators that is able totranslate the controlled mirror; and wherein the light source isconfigured to illuminate the mirrors with a time divisioncolor-multiplexed light beam.
 2. The apparatus of claim 1, wherein thelight source is configured to vary the light beam between having a firstcolor and having a second color at a frequency of greater than 20 Hertz.3. The apparatus of claim 2, comprising a processor being configured tooperate the MEMS actuators such that the mirrors of the array have afirst spatial distribution of translations when the light beam has thefirst color and have a different second spatial distribution oftranslations when the light beam has the second color.
 4. The apparatusof claim 1, further comprising: a planar viewing screen; and wherein theapparatus is configured to form an image on a viewing screen such that ahuman would observe the image to be temporally constant and have asuperposition of the colors of the coherent sources.
 5. The apparatus ofclaim 4, wherein the viewing screen non-specularly reflects lightincident thereon.
 6. The apparatus of claim 1, further comprising: abeam splitter located to redirect a light beam from the light sourcetowards the reconfigurable array; and a wave plate located between thebeam splitter and the reconfigurable array.
 7. The apparatus of claim 1,wherein the diameters of the mirrors are less than 20 times a centerwavelength of light produced by the light source; and wherein theprocessor is configured to operate the MEMS actuators such that thearray forms a first image of a first color on a viewing screen and formsa second image of a second color on the viewing screen such that thefirst and second images have the same size.
 8. A method of forming animage with a reconfigurable array of mirrors, comprising: configuringthe array by translating some of the mirrors such that distances of themirrors of the array from the reference plane have a non-uniform spatialdistribution; illuminating the configured array with a coherent lightbeam such that part of the light beam is reflected off the array and isprojected on a planar viewing screen.
 9. The method of claim 8, whereinthe illuminating causes the viewing screen to non-specularly reflectpart of the light projected thereon.
 10. The method of claim 8, furthercomprising: then, reconfiguring the array by translating some of themirrors such that distances of the mirrors of the array from th thereference plane have a new non-uniform spatial distribution;re-illuminating the reconfigured array with a coherent light beam of adifferent color such part of the light beam of the different color isprojected on the planar viewing screen.
 11. The method of claim 10,further comprising: during each of a series consecutive of time periods,repeating the acts of configuring, illuminating, reconfiguring, andre-illuminating.
 12. The method of claim 11, wherein each of the periodsis less than 1/20 seconds long.
 13. The method of claim 12, wherein themirrors have largest diameters that are smaller than 20 times awavelength of light of the light beam.
 14. The method of claim 9,wherein the mirrors have largest diameters that are smaller than 20times the a wavelength of light of the light beam.